The metric to watch. Possible values:
* host.cpu.percent - On a scale of 0 to 100, the percent CPU a guest is using.
* host.network.backend.in and host.network.frontend.in - The network bytes-per-second incoming on the interface
of either the frontend or backend network.
* host.network.backend.out and host.network.frontend.out - The network bytes-per-second incoming on the interface
of either the frontend or backend network.